I have used the DLink 2200 APs for about 6 months now with good Success. They have released firmware version 2.0 which is a signifigant improvement over version 1.0. You need to determine if the problem is the AP or the MAC. I am a PC guy so I can only offer PC based tools and tips. You can use Netstumbler to measure signal strength. It will also tell you which BSSID you are currently connected to. If you take your netstumbler laptop (or PDA) to the problem area, and identify the problem BSSID. Then you can log on to the individual APs and look at the config and the stats. You may have an AP with a failing radio. I hope your APs are alternating betwen channels 1, 6 and 11. You should pay attention to the signal strength and the connected speed when looking for problems. DLINK has a very complete spec sheet that shows the signal level required for each of the data rates. If your data rate is lower than the spec sheet says it should be then there may be noise or interference in the area on that channel. If clients are connecting several speed levels below the signal strength on the spec sheet you probably have a problem with that AP or that channel. This is all I can think of at this time.

If you really want to get detailed, get a sniffer trace over the air. Knoppix 3.9+ and the right wireless NIC card can provide you a free wireles sniffer using Tethereal.